U.S. Pat. No. 4,675,358 discloses coating compositions employing phosphoric acid derivatives. Such compositions are not very hydrolytically stable. WO 96/08518 describes N-acyl aminomethylene phosphonates and derivatives and their use in waterborne coatings. Such compositions have limited compatibility with other binders and limited durability because of N-containing linkages. EP 391,230 discloses compounds which are the reaction product of an alpha-aminomethylene phosphonic acid with an epoxy group of a compound containing at least one epoxy group. Such reaction products have limited end use in water-borne compositions that are anionically stabilized since the amino group destabilizes the coating composition. U.S. Pat. No. 5,151,125 discloses aqueous metallic coating compositions comprising acrylic copolymers to reduce the gassing of aluminum flakes. Such compositions have limited hydrolytic stability and limited effectiveness.
We have found that hydrophobic-substituted phosphonic acid derivatives which are reaction products with hydroxy-functional copolymers are particularly effective for inhibiting the corrosion of metallic pigments in water-borne compositions. The invention concerns an aqueous coating composition comprising a stabilized film-forming polymer, a metallic pigment and a compound which is the reaction production of a, b and optionally c, wherein:
(a) is 5 to 95 weight percent of a phosphonic acid derivative consisting of one or both of 
wherein R and Rxe2x80x2 are selected from the group consisting of an aliphatic, cycloaliphatic and aromatic substituent having 1 to 25 carbon atoms and optionally up to 10 heteroatoms of at least one of oxygen, phosphorus and silicon;
(b) is 5 to 95 weight percent of a hydroxy-functional addition polymer with a weight average molecular weight of 1000 to 30,000; and
(c) is 0 to 90 weight percent of a compound having one hydroxy-functional group.
Representative examples of phosphonic acid group-containing compounds are vinyl phosphonic acid, propyl and octyl phosphonic acid, and hydroxyethane diphosphonic acid.
Representative examples of hydroxy-functional polymers are addition copolymers of methacrylates, vinylaromatics, acrylates, acrylamides/methacrylamides and their derivatives, acrylonitrile, methacrylonitrile, allylalcohol, maleates, itaconates and vinyl monomers. The copolymers are hydroxy and/or acid functional. Typical comonomers useful to obtain the hydroxy-functionality are 2-hydroxy ethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxy propyl acrylate, 2-hydroxy propyl methacrylate, allyl alcohol, 1,4-butenediol, 4-hydroxybutyl acrylate, bis-hydroxy ethyl maleate and the like. Preferred hydroxy-functional copolymers are copolymers of styrene and allyl alcohol. A preferred phosphonic acid derivative is propyl and octyl phosphonic acid.
The hydroxy functional copolymers are typically prepared in a free-radical initiated process using peroxy or azo initiators. The reaction between the hydroxy functional copolymer and the vinyl and/or alkyl phosphonic acid can be run in a fusion or solvent process where water is distilled.
The reaction products of the phosphonic acid derivatives and the hydroxy-functional copolymers are used to treat metallic pigments before the final blend is inverted in water. The final metallic pigment dispersions are stable from gassing.
Preferred coating compositions are the following:
where the reaction product as corrosion inhibitor is used in the form of a salt;
where the phosphonic acid derivative is propyl, octyl or vinylphosphonic acid;
where the hydroxy-functional product is based on a styrene-allyl copolymer;
where the corrosion inhibitor based on phosphonic acid has an acid value of 25 to 400 mgKOH/g;
where the composition is anionically or non-ionically stabilized, and where the film-forming polymers are anionically stabilized vinyl, acrylic and/or urethane emulsions.
Preferred phosphonic acid group-containing reaction products are those formed from one or more of vinyl phosphonic acid, propyl and octyl phosphonic acid, and hydroxyethane diphosphonic acid with one or more hydroxy-functional copolymers of styrene and allyl alcohol. Preferably, R and Rxe2x80x2 have up to 12 carbon atoms and effect good hydrophilic-lipophilic balance (HLB).